You cannot cancel the order by your self through online portal,
However you should contact the seller immediately and request him to cancel the order before its shipped.
once the order is shipped it cannot be cancelled.
One experience, I ordered kingston 32gb microsd card, i had opened another option sandisk32gb in another tab which i had earlier decided to order. As i was distracted with all my friends talking to me about tech stuff i mistakenly ordered kingston and also forgot to enter 150 bucks discount coupon(which i only realised after getting message of amount deduction on phone by bank). So i called the seller immediately(within 13 min). To my surprise the seller had already shipped it within 13min and i had also received the shipment details(bluedart). Well i did not know weather to be happy or sad. Appreciate the sellers responce though.
